The present invention relates to a method for plasma cutting a workpiece by means of a plasma cutting system including a plasma current source and a plasma torch which comprises an electrode and a nozzle that is a small distance from the electrode at a lower end of the plasma torch in order to form a plasma chamber there-between.
By way of plasma, a conductive gas is used which can be heated to a high temperature level and which consists of positive and negative ions as well as excited and neutral atoms and molecules.
By way of plasma gas, different gases, for example monoatomic argon and/or diatomic gases hydrogen, nitrogen, oxygen or air, are used. These gases ionise and dissociate through the energy of an arc. The arc which is tapered through a nozzle is then described as a plasma jet.
The plasma jet can be greatly influenced in its parameters by the design of the nozzle and electrode. These parameters of the plasma jet are, for example, the jet diameter, the temperature, the energy density, and the flow speed of the gas.
During plasma cutting, for example, the plasma is tapered through a nozzle which can be gas-cooled or water-cooled. Energy densities of up to 2×106 W/cm2 can thereby be achieved. Temperatures of up to 30,000° C. arise in the plasma jet which facilitate, in association with the high flow speed of the gas, very high cutting speeds on the materials.
Plasma cutting systems generally consist of at least one current source, a plasma torch, and a gas supply.
Due to the high thermal load on the nozzle, the nozzle is generally constructed of a metal material, preferably copper, due to its high electrical conductivity and heat conductivity. The same applies to the electrode holder which can also be constructed of silver. The nozzle is then used in a plasma torch, the main components of which are a plasma torch head, a nozzle cap, a plasma gas conveying part, a nozzle, a nozzle holder, an electrode receiving element, an electrode holder with emission insert and, in the case of modern plasma torches, a nozzle protection cap holder and a nozzle protection cap. The electrode holder fixes a sharp electrode insert made of tungsten which is suitable for the use of non-oxidising gases as plasma gas, for example, an argon-hydrogen mixture. A flat electrode, the emission insert of which consists of zirconium or hafnium for example, is also suitable for the use of oxidising gases as plasma gas, such as, for example, air or oxygen. Zirconium can be used for oxygen-containing plasma gas. Due to its better thermal properties, however, hafnium is better suited as its oxide is more temperature-resistant.
In order to achieve a long lifespan for the nozzle and the electrode, cooling is often effected with a liquid, for example water, but it can also be carried out with a gas. In this respect a distinction is made between liquid-cooled and gas-cooled plasma torches.
In order to achieve a long lifespan of the electrode, the high temperature material is incorporated as an emission insert into the holder which is then cooled. The most efficient type of cooling is liquid cooling. The arc burns between the emission insert of the electrode and the nozzle and/or the workpiece to be cut. During operation the emission insert is gradually worn away and a hole is drilled in the electrode. It frequently also arises that the arc goes on to the electrode holder and destroys it. This occurs particularly when the emission insert has burnt back deeper than 1 mm, and has the effect of damaging the electrode which must then be replaced.
The current sources used for plasma cutting are predominantly direct current sources with a greatly falling characteristic curve or constant current curve. Fluctuations of the cutting voltage caused by the process thereby have no effect or very little effect upon the cutting current. These fluctuations are caused, for example, by different torch distances from the workpiece, by fluctuations in the gas supply, and through wear of components of the plasma torch.
Examples for current sources with a greatly falling characteristic curve are scattering field or scattering core transformers with a subsequently arranged rectifier. The falling characteristic is produced here through the arrangement of the coils of the transformer.
In the case of modern direct current sources, the constant current curve is realised through the regulation of the cutting current using electronic components, e.g. thyristors and transistors. In principle it is possible to distinguish here between network-guided current sources and current sources with increased frequency.
Network-guided current sources are those in which the intervention time of the regulation is determined by the frequency of the voltage of the current supply network and its zero-crossing. A variant is a transformer with a subsequently arranged thyristor-controlled rectifier. The minimum intervention time possible for the regulation in the rectifier amounts, according to the circuit variant, to between 6.6 ms with a 3-pulse bridge circuit and 1.6 ms with a 12-pulse bridge circuit.
Current sources with increased frequency have substantially lower intervention times for the regulation as the frequency is clearly higher than the frequency of the network voltage. The intervention times, depending upon the frequency of the current source, lie between 100 μs and 5 μs.
One such variant configuration includes a transformer, an unregulated diode rectifier, and a subsequently arranged transistor switch, also described as a chopper, which regulates the current. A further variant, often called an inverter, consists of an unregulated diode rectifier, an inverter, a transformer, and a diode rectifier. A frequency of between 10 and 200 kHz is used as a frequency for the chopper and the inverter.
According to the prior art—apart from undesired but unavoidable noise or undesired harmonics—a direct current which is as even as possible is required for good cutting quality and long lifespan of the parts of the plasma torch that are subject to wear. For this, mostly inductive structural units (restrictors) are arranged in the current circuit of the cutting current in order to reduce the ripples caused by the current network and the switching processes of the current source.
Current sources which work with a higher frequency can, in contrast with network-guided current sources, regulate the ripples of the direct current caused by the frequency of the network voltage as the frequency of the current source is clearly higher than the frequency of the network voltage. Often such current sources are only available in a limited power, e.g. 10 to 20 kW. For this reason, a plurality of current sources are arranged in parallel. Network-guided current sources and current sources with increased frequency can be arranged in parallel.